Thermal transfer systems have been developed to obtain prints from pictures that have been generated electronically, for example, from a color video camera or digital camera. An electronic picture can be subjected to color separation by color filters. The respective color-separated images can be converted into electrical signals. These signals can be operated on to produce electrical signals corresponding to various colors, for example, black, cyan, magenta, or yellow. These signals can be transmitted to a thermal printer. To obtain a print, a colored dye-donor layer, for example, black, cyan, magenta, or yellow, of a dye-donor element can be placed face-to-face with a dye image-receiving layer of a receiver element to form a print assembly, which can be inserted between a thermal print head and a platen roller. A thermal print head can be used to apply heat from the back of the dye-donor element. The thermal print head can be heated up sequentially in response to the various color signals. The process can be repeated as needed to print all colors, and a laminate or protective layer, as desired. A color hard copy corresponding to the original picture can be obtained. Further details of this process and an apparatus for carrying it out are described in U.S. Pat. No. 4,621,271 to Brownstein.
Thermal transfer works by transmitting heat through the dye-donor element from the back-side to the dye-donor layer. When the dyes in the dye-donor layer are heated sufficiently, they sublime or diffuse, transferring to the adjacent dye-receiving layer of the receiver element. The density of the dye forming the image on the receiver can be affected by the amount of dye transferred, which in turn is affected by the amount of dye in the dye-donor layer, the heat the dye-donor layer attains, and the length of time for which the heat is maintained at any given spot on the dye-donor element.
At high printing speeds, considered to be 2.0 ms/line or less, the print head undergoes heat on/off cycles very rapidly. This generated heat must be driven through the dye-donor element very rapidly to effect the dye transfer from the dye-donor layer to the receiver. Each layer in the dye-donor element can act as an insulator, slowing down the heat transfer through the layers of the dye-donor element to the receiver. Because of the short heat application time, any reduction in heat transfer efficiency results in a lower effective temperature in the dye-donor layer during printing, which can result in a lower transferred dye density. It is known to overcome the low print density associated with shorter line times by increasing the print head voltage, increasing the relative amount of dye in the dye-donor layer, or a combination thereof. Applying higher print head voltages can decrease the lifetime of the thermal print head, and requires a higher power supply, both of which increase cost. Increasing the relative amount of dye in the dye-donor layer increases costs, as well as increasing the chance of unwanted dye transfer, such as during storage of a dye-donor element.
Another problem exists with many of the dye-donor elements and receiver elements used in thermal dye transfer systems. At the high temperatures used for thermal dye transfer, many polymers used in these elements can soften and adhere to each other, resulting in sticking and tearing of the dye-donor and receiver elements upon separation from one another after printing. Areas of the dye-donor layer other than where dye was image-wise transferred to the receiver can adhere to the dye image-receiving layer, causing print defects ranging from microscopic spots to sticking of the entire dye-donor layer on the receiver. This is aggravated when higher printing voltages, resulting in higher temperatures, are used in high speed printing. Another problem with high speed printing is that the more rapid physical motion of the dye-donor/receiver assembly results in higher peel rates between the dye-donor element and the receiver element as they are separated after printing, which can aggravate sticking of the dye-donor and receiver elements.
Various binders and plasticizers have been used in thermal dye-donor elements. JP 62-094382A discloses a mixture of a polycaprolactone and a polyester for use as a binder in a thermal dye-donor. U.S. Pat. No. 5,750,465 also discloses use of a specific polyester as a plasticizer for use in a thermal dye-donor.
U.S. Pat. No. 4,732,815 describes a thermal dye transfer system wherein polycaprolactone is a resin in a filling layer, which is coated over a hot melt ink layer. It is desired that the hot melt ink layer have a very low melt viscosity so that complete transfer of the entire hot melt ink layer (binder and ink) to the dye receiving layer will occur. The filling layer is also completely transferred onto the dye receiving layer surface. Such a system, commonly referred to as a hot wax transfer system, mass transfers the dye-donor layer onto the dye receiving layer by the heat of the thermal print head. This does not produce a gradation of densities and is not appropriate for dye diffusion or sublimation thermal dye transfer systems, where it is desired that only the dye transfer and diffuse into the dye receiving layer to obtain a continuous tone (gradation of densities) image.
There is a need in the art for a means of providing one or more of the following advantages using a dye-donor element: 1) maintaining or increasing print density, such as by increased dye transfer efficiency; 2) maintaining or reducing power to the print head; 3) reducing or eliminating dye-donor/receiver sticking; and 4) increasing print speed.